The present invention relates to a process for producing an oxygen-containing cyclic fluoro compound. More particularly, it relates to a process for producing oxygen-containing cyclic fluoro compounds such as fluorine-containing lactones and fluorine-containing cyclic ethers by reacting an .alpha., .omega.-diiodopolyfluoroalkane with fuming sulfuric acid. Oxygen-containing compounds such as fluorine-containing lactones and fluorine-containing cyclic ethers are useful as intermediates for various fluorine-containing compounds and compounds having special characteristics. For example, the fluorine-containing lactones can be easily converted to the corresponding fluorine-containing dicarboxylic acid derivatives by reaction with a nucleophillic reagent. These derivatives can be effectively used as polybasic acids for producing fluorine-containing condensation polymers such as polyamides and polyesters. On the other hand, the fluorine-containing lactones are intermediates which are useful for the synthesis of various fluorine-containing vinyl monomers, especially fluorine-containing vinyl ethers, and they possess excellent characteristics which enable their use as lubricants, fiber processing agents, and the like. The fluorine-containing cyclic ethers have excellent characteristics as dielectrics and heat transfer media. However, these useful oxygen-containing cyclic fluoro compounds have been hard to produce in high yield. Moreover, the processes necessary for producing these materials are very complicated, and a satisfactory, industrial process for producing these compounds is not known. Thus, for example, fluorine-containing lactones have been produced by heating silver perfluoroglutarate at 125.degree. C. in the presence of iodine (R. E. Banks et al.) However, the yield of the desired perfluoro- .gamma.-butyrolactone compound is only 8% or less (JCS(C) 1967, 2333). In another method of producing fluorine-containing cyclic ethers, an electrolyzing fluorination step has been used as a required step in the process whereby the yield was about 25% or less. This is also quite disadvantageous from an industrial viewpoint. A need, therefore, continues to exist for a method of smoothly and effectively producing useful oxygen-containing cyclic fluoro compounds such as fluorine-containing lactones, e.g. perfluorolactones and fluorine-containing cyclic ethers, e.g. perfluorocylic ethers in high yield.